1. Field of the Invention
This invention relates generally to surge protection devices for protecting telephone communications related equipment from power and transient surges. More particularly, the present invention relates to an improved dual stage current limiting surge protector system for protecting telecommunication equipment from power and transient surges occurring on the tip and/or ring conductors of transmission lines connected thereto. Specifically, the dual stage current limiting surge protector system of the instant invention includes a first pair of current limiting devices connected in series between corresponding input tip and ring terminal pins and opposite ends of a voltage suppressor and a second pair of current limiting devices connected in series between corresponding output tip and ring terminal pins and the opposite ends of the voltage suppressor.
2. Description of the Prior Art
As is generally well-known to those skilled in the telecommunications industry, modern telecommunications equipment is susceptible to transient surges such as those caused by lightning strikes and other voltage surges on the transmission lines. Accordingly, various types of surge protector circuits are known in the prior art which have been provided for connection to the incoming transmission lines so as to be normally non-operative but are rendered active when a voltage/current surge exceeds a predetermined limit for protecting the sensitive equipment.
A prior art search directed to the subject matter of this application in the U.S. Patent and Trademark Office revealed the following Letters Patent and application:
4,587,5885,854,7304,739,4365,956,2235,115,3685,978,1985,136,4606,430,1985,488,5346,556,3945,502,6126,560,0865,774,315
Japanese Patent No. 3,212,154 and U.S. Publication No. 2002/0048130.
In U.S. Pat. No. 4,587,588 to Richard Goldstein issued on May 6, 1986, there is disclosed a power line transient surge suppressor circuit which includes a fuse inserted in series with a surge-bypassing network in which the fuse blows in the event of a short-circuit failure of a regulator device in the network. A thermal cut-out is inserted in series with the suppressor circuit input. A current-limiting heater resistor is connected in parallel with the fuse and supplies heat to trip the thermal cut-out after the fuse blows, thereby terminating power output to the loads.
In U.S. Pat. No. 4,739,436 to Joseph P. Stefani et al. issued on Apr. 19, 1988, there is disclosed a voltage suppression circuit which employs two series connected metal oxide varistors and a power interrupter to provide suppression of all modes of voltage surge while providing protection against failure of either metal oxide varistor. The power interrupter consists of a current-limiting fuse.
U.S. Pat. No. 5,502,612 to Joseph c. Osterhout et al. issued on Mar. 26, 1996 teaches a secondary surge arrestor which includes a nonlinear voltage dependent resistive element and a fault current limiting fuses connected between the conductors and disposed within the casing. The fault current limiting fuse is formed of a fusible material surrounded by sand.
U.S. Pat. No. 6,430,017 to David A. Finlay, Sr. et al. issued on Aug. 6, 2002, teaches a transient voltage suppressor for an electrical circuit which includes a metal oxide varistor connected in series with a thermally responsive fuse and a current limiting fuse, all joined between hot and neutral conductors. A load is also connected between the hot and neutral conductors.
The remaining patents, listed above but not specifically discussed, are deemed to be only of general interest and show the state of the art in transient and voltage surge protection systems for preventing damage to electrical equipment.
Further, there is shown in FIG. 1 a prior art surge protector circuit that is also known to the applicant of the present invention. The surge protector circuit is comprised of a voltage suppressor and a pair of current limiting devices connected in series with the opposite ends of the voltage suppressor. Such prior art surge protector circuit of this type is required to meet or pass the primary test according to the specification set forth by UL Standards 497 and 497A. However, in order to accommodate the UL test where high rated level of current must be allowed to pass through the current limiting devices, the current limiting devices are selected to have a relatively high rated value, such as 350 ma for fuses or 160 ma for positive thermal coefficient (PTC) resistors. Unfortunately, the use of the current limiting devices having these high values can cause damage to some sensitive equipment during normal operating conditions when such high level currents are allowed to pass therethrough.
None of the prior art discussed above disclosed a dual stage current limiting surge protector system like that of the present invention which includes a first pair of current limiting devices, a voltage suppressor, and a second pair of current limiting devices. As a result, the instant surge protector system passes the primary test of the UL Standards 497 and 497A and still protects sensitive equipment from being damaged.